Program: GN-2018B-FT-112

The physics and chemistry of brown dwarf (BD) atmospheres are complex and resemble those of giant planets. Existing models are also complex, and can reproduce the energy distributions of BDs except for the 3.2- to 4.2-micron region where the calculated flux is 2 - 10 times too low for BDs cooler than 600K. This significantly hinders analyses of cold BDs which emit most of their energy in the mid-infrared, and also impacts JWST planning, for which BDs are prime targets. In cool atmospheres there is strong CH4 absorption at 3-microns and flux emerges from near the cold surface, at 4-microns the atmosphere is clear and flux emerges from deep and hot regions. Models show that the 3- to 4-micron region spans the largest range in atmospheric pressures of any spectral region. We take advantage of this by obtaining NIRI L' (3.8-micron) images of five 400-500K BDs that show intrinsic scatter in the W1(3.4)-[3.6] color. The scatter is not reproduced by any model and represents missing physics. Using the W1(3.4), [3.6] and L'(3.8) filters we will map the extreme atmospheric environment that gives rise to the 3- to 4-micron flux and explore model enhancements such as additional convection, clouds or unusual chemical abundances, that may solve the mystery of the missing 4-micron flux.